In order to connect distribution transformers to utility power lines, it is necessary to provide an electrically conductive path through the wall of a sealed and grounded case to an inside assembly. Such connections are typically accomplished by means of a device known as a feed-through bushing. Feed-through bushings are constructed of two types of materials, namely, an electrical conductor and an electrical insulator. A typical bushing comprises an inner electrical conductor of copper or other electrically conductive material surrounded by an electrical insulator of porcelain or an equivalent insulating material. If a bushing is designed to withstand high voltage applications, (14,000 VAC and higher), it is necessary to provide the bushing with an inner cavity surrounding the conductor and to fill the cavity with an additional insulator of high dielectric strength, such as transformer oil. In the event this additional dielectric filler is omitted or has partially leaked from the bushing, an electrical failure of the bushing in high voltage applications may occur.
Another problem may exist with this arrangement which can cause the generation of radiated noise in the form of radio frequencies. This radiation is usually of sufficient magnitude as to interfere with radio frequency reception (TV, radio, etc.). Such radiation is produced in the bushing by the effect of a high voltage field on air trapped inside the inner cavity of the bushing. A bushing operating under this condition will eventually fail electrically. The present invention solves the two above-described problems in the prior art devices.